This invention relates to a method of producing an abrasive product containing cubic boron nitride and cemented carbide.
Cemented carbide is a material which is used extensively in industry for a variety of applications, both as an abrading material and as a wear resistant material. Cemented carbides generally consist of suitable carbide particles such as tungsten carbide, tantalum carbide or titanium carbide, bonded together by means of a bonding metal such as cobalt, iron or nickel, or an alloy thereof. Typically, the metal content of cemented carbides is about 3 to 35% by weight. They are produced by sintering the carbide particles and the bonding metal at temperatures of the order of 1400° C.
At the other end of the spectrum, ultrahard abrasive and wear resistant products are found. Diamond and cubic boron nitride compacts are polycrystalline masses of diamond or cubic boron nitride particles, the bonding being created under conditions of elevated temperature and pressure at which the ultrahard component, i.e the diamond or cubic boron nitride, is crystallographically stable. Polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) can be produced with or without a second phase or bonding matrix. The second phase, when provided, may be, in the case of diamond, a catalyst/solvent such as cobalt, or may be a carbide forming element such as silicon. Similar sintering mechanisms are utilised in PCBN synthesis with various carbides, nitrides and borides being common second phases.
PCD and PCBN have a far higher wear resistance than cemented carbides, but tend to be somewhat brittle. This brittleness can lead to edge chipping of the working surface which can present a problem in applications where fine finishes are required. Furthermore, ultrahard products such as PCD and PCBN can generally not be directly brazed onto a metallic support. They are therefore often sintered in combination with a cemented carbide substrate. The bi-layered nature of such ultrahard products can be problematic in terms of thermo-mechanical stresses between the two materials: differential expansion and shrinkage on heating and cooling due to different thermal expansion coefficients and elastic moduli can lead to crack formation or unfavourable residual stresses if the substrate and the ultrahard products are too dissimilar. Another potential problem of such bi-layered materials is that of undercutting, i.e. preferential wear of the less abrasion resistant carbide support. Further, machining of ultrahard products is difficult and costly, where carbide products can be relatively easily ground to the final geometry.
Efforts have been made to solve some of these problems.
JP-A-57 116 742 discloses the preparation of a modified cemented carbide under hot pressing conditions, i.e. temperatures of the order of 1400° C. to 1500° C. with little or no pressure being applied. These are not conditions at which cubic boron nitride is crystallographically stable.
European Patent No 0 256 829 describes a method of producing an abrasive and wear resistant material comprising a mass of carbide particles, a mass of cubic boron nitride particles and a bonding metal or alloy bonded into a coherent, sintered form, the cubic boron nitride particle content of the material not exceeding 20% by weight and the material being substantially free of hexagonal boron nitride, which comprises contacting appropriate amounts of a mass of carbide particles and a mass of cubic boron nitride particles with a bonding metal or alloy and sintering the particles and metal or alloy under temperature and pressure conditions at which the cubic boron nitride is crystallographically stable.